The present invention relates to a method of producing fluoropolymer films at high productivity and to films produced using the method.
The production of thin plastic films has generally been accomplished using one or more of three processes: melt extrusion, casting from solutions or organosols, and casting from aqueous dispersions. Melt extrusion of films is generally preferred to casting since it does not require the removal of an organic solvent, water, or surfactants. It therefore produces a very clean film and typically is characterized by high productivity. Melt extrusion cannot, however, be used for all materials.
Casting methods are preferred if the required time at extrusion temperature is sufficient to result in thermal or oxidative degradation of the polymer. Casting is also preferred when the melt viscosity of the polymer is sufficiently high to make extrusion either technically impossible or economically impractical.
In the case of fluoroplastics, all three processes are used to produce films, with the choice of process largely depending on the monomer content of the polymer. The most common monomers presently employed to produce fluoroplastics include tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), vinylidene fluoride (VF.sub.2), and vinyl fluoride (VF). All of these are available as homopolymers; i.e., PTFE (e.g. "Teflon"), PCTFE (e.g. "KelF"), PVF.sub.2 (e.g. "Kynar"), and PVF (e.g. "Tedlar"), respectively. PCTFE and PVF.sub.2 are melt extrudeable as thin films with some difficulty due to the fact that the time/temperature history during extrusion is near to that which could result in polymer degradation at the severe shear rate of melt extrusion. This condition can be further aggravated in the presence of certain fillers. PVF film cannot be produced by melt extrusion due to thermal instability and thus is produced by a casting process and subsequently is biaxially stretched. Homopolymer PTFE cannot be practically melt extruded at all due its extraordinarily high melt viscosity.
In order to overcome such problems in the case of melt extrusion of these homopolymers, copolymers of these monomers have been developed which are generally lower in melting temperature and melt viscosity at extrusion temperatures. This allows extrusion of the polymers at temperatures at which no significant thermal degradation occurs. Consequently, fluoropolymer films are most generally based upon such readily extrudeable copolymers. These include copolymers of TFE with hexafluoropropylene, e.g., "Teflon" FEP, or with perfluoroalkyl vinyl ethers, e.g., "Teflon" PFA, or with ethylene, e.g. "Tefzel" ETFE. Similarly, copolymers of CTFE include those with vinylidene fluoride or hexafluoropropylene, e.g., "Kynar", as well as with ethylene, e.g., "Halar". Terpolymers of these basic monomers are also known and used in extrusion.
Since pure PTFE cannot be melt extruded, as mentioned above, other processes have been developed for film production. One such method involves the skiving of thin film from a molded and sintered billet. Another involves the casting of an aqueous dispersion onto a metallic carrier. The deposited resin is subsequently stripped from the carrier to yield a very high quality film relative to the skived films.
A casting process for PTFE is described in U.S. Pat. No. 2,852,811 issued to John V. Petriello in 1958, which is incorporated herein by reference. In summary, this process involves continuously depositing a layer of a PTFE dispersion onto a metal carrier, drying the coated carrier and then sintering the dried coating. These steps are then repeated until a film of the desired thickness had been formed. The film is then stripped from the carrier. U.S. Pat. No. 2,852,811 stresses the importance of the nature of the carrier belt used in the casting process. Thus, highly polished, corrosion resistant metal carrier belts have been used in subsequent casting efforts.
Cast PTFE films exhibit virtually no mechanical anisotropy, and have substantially higher tensile strength, elongation, and dielectric breakdown strength than skived PTFE films. Unfavorable process economics, however, have prevented a wide acceptance of casting as a method for making fluoropolymer films. Among the factors affecting the economics are the properties of the metal carrier belts. These belts are fairly rigid and heavy, and thus require a special tracking mechanism to drive the belt through the apparatus. This essentially fixes the width of the material produced, causing a loss in versatility.
The casting process as described by Petriello also suffers as a result of low productivity. In an effort to elaborate upon the significant process parameters affecting film quality, investigations were sponsored by the Aeronautical Systems Division of the United States Air Force between 1955 and 1962 which resulted in the publication of a report entitled "Production Refinement of Very Thin Teflon Film." This publication emphasized the importance of the dispersion characteristics and line speed as each can significantly affect the quality of the cast film. Specifically, the Air Force study observed that the quality of film produced by the casting method deteriorates very rapidly at line speeds above 3 feet per minute. (See P. 19 "Production of Very Thin Teflon Film"). Productivity of film manufacture at such slow rates is in general prohibitively costly: even simple, monolithic cast films of PTFE must be sold at four to five times the price of skived PTFE or two to three times that of extruded FEP to be economically attractive. This has led to very minimal acceptance of cast films in the marketplace and has been the major cause of the lack of continued research over the past decades into processes for casting fluoropolymer films.
Additionally, the very low critical cracking thickness of most fluoropolymer dispersions suggest that thicker individual lamellae within any given film cannot be achieved to even partially offset the poor productivity associated with very low linear line speed.
It is of interest to note, however, that all of the previously mentioned fluoroplastic homopolymers and copolymers are available as aqueous dispersions and can be used to produce cast films. Moreover, the casting process potentially offers several distinct advantages over the extrusion process for producing films. The casting process inherently is a multi-layering process; thus, multi-layer film production by casting methods avoids the intrinsic problems and substantial unit investment which would be associated with coextrusion or extrusion coating of fluoropolymers. PTFE films with surface(s) of fluorinated ethylene propylene (FEP) or perfluoroalkoxy resins (PFA) are available commercially from casting equipment. Additionally, the casting of alloyed fluoropolymers, including both thermoplastic and elastomeric polymers and which may optionally incorporate metal, mineral, or ceramic additives to modulate chemical, optical, electrical, and magnetic transport properties of film is facilitated by the casting process in both monolithic (uniform composition) and complex (non-uniform composition) film format. Such films are described in commonly assigned U.S. patent application Ser. Nos. 600,002 and 908,938, and U.S. Pat. Nos. 4,555,543 and 4,610,918, all four of which are incorporated herein by reference. Most importantly, such a process permits one to combine in a single layer, or in sequential layers, polymers with widely different melting temperatures and degradation temperatures since the time/temperature history of the film as it is processed can be kept much shorter than that characteristic of melt extrusion.
In short, the casting process is an inherently much more powerful method than the extrusion process for producing high quality films with a far larger number of compositional degrees of freedom. It is an object of the present invention to provide a method for the production of fluoropolymer films in which the relationship between productivity and film quality is dramatically altered such that one can economically take advantage of this superiority. The products of this process could enjoy significant use in electrical and electronic applications as well as in selective membranes and other chemical applications.